Power units that drive rail-bound cars along a set of tracks are commonly driven by electric traction motors. A diesel-powered engine (main engine) is provided on the power unit to drive a main generator that produces the electricity used by the traction motors. Other electrical components and accessories, such as air compressors, cooling fans, heating/ventilation/air-conditioning (HVAC) units, and control systems on the power unit and on other railcars coupled thereto are powered by an accessory power unit (APU). The APU typically includes a dedicated diesel- or gas-powered engine and an auxiliary generator driven thereby such that the APU can operate independently from the main engine. In some of these systems, power from the APU can be diverted from the accessories to the traction motors in case of main generator failure. It is also known to employ the main engine to drive the auxiliary generator for the accessories rather than utilizing an APU with a separate dedicated engine.
Problems exist with the known power unit configurations. For example, when the main engine is employed to drive the auxiliary generator, failure of the auxiliary generator leaves the power unit with no accessory power; and failure of the main generator leaves the power unit immobile. Because many of the accessories are required in order to operate the power unit, either failure can prevent further operation. In configurations utilizing an APU with a dedicated engine and generator, accessory power is lost if the APU fails. In the event of a main engine failure or main generator failure, the power output of the APU can be diverted to power the traction motors. In either configuration, the power unit is at best left without accessory power and at worst left without any power. There is a need in the art for a power unit configuration and system that provides backup power for both traction motors and accessories during failure of the main generator and/or the auxiliary generator.